I'm not sure if you really wanted to use mediatorMap
.registerMediator(), or if you mistook it for
mediatorMap.mapView(). Could you give us more info about your use
case? Why do you need to use registerMediator, i.e. what do you
want to achieve exactly?

and then we add the view to the stage:
someContainer.addElement(view);

But that's exactly what createMediator() does.
So, instead of the above you could do this:

Map the view to the mediator in you config.

mediatorMap.mapView(SomeView, SomeMediator);

Create a new SomeView and a new SomeMediator

var view:SomeView = new SomeView();
mediatorMap.createMediator(view);

[EDIT]

Usually, that's only needed for mediating components like
Popups. Regular views are mediated automatically, when they are
added to the contextView's stage, provided you mapped them to a
mediator, and autoCreate is set to true :
mediatorMap .mapView(viewClassOrName:*, mediatorClass:Class,
injectViewAs:*=null, autoCreate:Boolean=true,
autoRemove:Boolean=true):void